With regard to the next generation wireless communication system, there is a fear that the frequency resources will be exhausted due to the high speed transmission and the diversified system. In recent years, the cognitive radio method, which can carry out the most suitable communication autonomously through recognizing a surrounding wireless environment and user's needs, has been studied. The dynamic spectrum access of the cognitive radio method, by which a frequency band assigned to an existing wireless system can be used secondarily by another wireless system, receives much attention from a view point of the efficient use of the frequency resources. Specifically, according to the dynamic spectral access, a vacant frequency band within a frequency band assigned to a first system, which is the existing wireless system, is used by a second system, which is a new wireless system, without the second system disturbing communication of the first system.
FIG. 22 is a configuration diagram of a general communication system for describing the dynamic spectrum access. According to the communication system, an uplink or a downlink of the second system uses the same frequency band as an uplink or a downlink of the first system uses respectively.
A system shown in FIG. 22 includes a first system 2210 and a second system 2220. The first system 2210 includes a first system base station 2211, a first system mobile station 2212 and a first system mobile station 2213. The first system base station 2211 sends and receives data with the first system mobile station 2212 and the first system mobile station 2213.
The second system 2220 includes a second system base station 2221, a second system mobile station 2222 and a second system mobile station 2223. The second system base station 2221 sends and receives data with the second system mobile station 2222 and the second system mobile station 2223.
The standardized specification IEEE802.22 WRAN is exemplified as an example, which applies the dynamic spectral access, in addition to the example shown in FIG. 22. Here, IEEE is the abbreviation of Institute of Electrical and Electronic Engineers. Moreover, WRAN is the abbreviation of Wireless Regional Area Network. IEEE802.22 is the standardized specification in USA on the system in which the fixed wireless access system as the second system uses a vacant channel in the frequency band of the ground-wave broadcasting and the wireless microphone as the first system.
Next, an interference causing avoiding technology related to the dynamic spectrum access will be described. FIG. 23A and FIG. 23B are schematic diagrams describing a general interference causing avoiding transmission method. In FIG. 23A and FIG. 23B, the horizontal axis indicates frequency and the vertical axis indicates power density. That is, FIG. 23A and FIG. 23B indicate a frequency band which the first system uses, and a spectrum which the second system has. FIG. 23A show an image of the spectrum to which the interference causing avoiding technology has not been applied yet. FIG. 23B shows an image of the spectrum to which the interference causing avoiding technology has been applied.
As a principle, it is necessary that the second system communicate without disturbing communication of the first system. Therefore, it is necessary that the second system sends data, which has a spectrum 2302-1 and a spectrum 2302-2, with avoiding to cause an interference to frequency bands 2301-1, 2301-2 and 2301-3 which the first system uses. However, as shown in FIG. 23A, the actual sending spectrum includes leak power which leaks outside a sending frequency band. Therefore, there is a fear that a partial spectrum of the second system will cause interference to the first system. Here, it is possible to avoid the interference to the first system through arranging a guard frequency band which makes a distance far enough from the frequency band which the first system uses. However, in the case that the enough guard frequency band is arranged, there is another fear that efficiency of using the frequency band will be degraded.
As it is clear from the above description, in the cognitive radio method which makes it possible that a plurality of systems share the identical frequency band, it is important that the second system sends data without degrading efficiency of using the frequency band of the second system and with avoiding the interference to the first system from being caused. In particular, in the case that the second system adopts a wireless access method based on OFDM, some kind of measure of avoiding the interference from being caused is needed since the leak power, which leaks outside the frequency band, becomes large due to the side lobe component of the subcarrier. Here, OFDM is the abbreviation of Orthogonal Frequency Division Multiplexing.
As an interference causing avoiding transmission method which avoids the interference from being caused in the first system, the digital filter method, the null regeneration method, the gauss type multi-carrier method, the subcarrier weighting method, the Time windowing method and the AIC method are exemplified. Here, AIC is the abbreviation of Active Interference Cancellation. According to the digital filter method, the spectrum is shaped by use of a FIR (Finite Impulse Response) filter or an IIR (Infinite Impulse Response) filter. According to the null regeneration method, FFT (Fast Fourier Transform) is carried out after a plurality of OFDM symbols are combined, and IFFT (Inverse Fast Fourier Transform) is carried out after the null subcarrier replacement. The gauss type multi-carrier system is a multi-carrier transmission method with which a spectrum is shaped in a gauss pulse. According to the subcarrier weighting method, each symbol, which is converted into a subcarrier signal, is weighted. According to the Time windowing method, an OFDM symbol is shaped in the time domain. According to the AIC method, a tone is generated in order to cancel a component which leaks outside the frequency band. The Time windowing method and the AIC method out of these methods have good compatibility with the commercialized existing wireless system, and have suitableness for restraining dynamically the in-band interference and the out-band interference from being caused in response to an surrounding wireless environment. Hereinafter, the Time windowing method and the AIC method will be described.
According to the Time windowing method which is disclosed in the non-patent document 1, the OFDM symbol is shaped into a raised cosine roll off waveform or the like at a sending end. As a result, it is possible to reduce discontinuity between the OFDM symbols and to make the side lobe component of the subcarrier small. The Time windowing method has a feature that it is possible to reduce power, which exists outside the frequency band, with a simple configuration.
FIG. 24 is a block diagram of a wireless sending apparatus which is disclosed in the patent document 1 and which reduces the power, which exists outside the frequency band, with the Time windowing method. This wireless sending apparatus includes a data mapping unit 2401, IFFT units 2402-1, 2402-2 and 2402-3, CP adding units 2403-1, 2403-2 and 2403-3, wave form shaping units 2404-1, 2404-2 and 2404-3, an adding unit 2405, a digital-to-analog conversion unit 2406 and an analog unit 2407. According to the patent document 1, a plurality of subcarriers are divided into modulation groups, and IFFT and the CP addition are carried out for each modulation group. Furthermore, each modulation group is multiplied with a waveform characteristic whose degree in the waveform shaping is different for each modulation group, and afterward the modulation group signals are added. As a result, it is possible to reduce the power which exists outside the frequency band of the modulation group of the second system near the frequency band used by the first system, and consequently it is possible to restrain the interference from being caused.
FIG. 25 is a schematic diagram describing an operation of the AIC method disclosed in the non-patent documents 2 and 3. FIG. 25, whose horizontal axis indicates frequency and the vertical axis indicates power density, shows that the second system sends spectra 2502-1 and 2502-2 on both sides of a frequency band 2501 which is used by the first system. The AIC method reduces the power, which exists outside the frequency band of the second system, through arranging dedicated tones 2503-1 and 2503-2 for cancelling a leak component of the second system's spectrum which leaks inside the frequency band used by the first system. A case that two AIC tones are arranged outside the frequency band 2501 which is used by the first system is exemplified in FIG. 25. The AIC method has a feature that it is possible to form a notch whose depth is about −30 dB to −100 dB, and consequently it is possible to reduce significantly the power which exists outside the frequency band.